Wafer inspection systems help a semiconductor manufacturer increase and maintain integrated circuit (IC) chip yields by detecting defects that occur during the manufacturing process. One purpose of inspection systems is to monitor whether a manufacturing process meets specifications. The inspection system indicates the problem and/or the source of the problem if the manufacturing process is outside the scope of established norms, which the semiconductor manufacturer can then address.
Evolution of the semiconductor manufacturing industry is placing ever greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions are shrinking while wafer size is increasing. Economics is driving the industry to decrease the time for achieving high-yield, high-value production. Thus, minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for the semiconductor manufacturer.
Detecting that a defect is in a test image can be important to a semiconductor manufacturer. Defects can be detected by comparing an image of a wafer to a reference image. For example, “double detection” can be used to detect defects. Using this double detection technique, the defect needs to be detected in two difference images. Each of these two difference images subtracts the test image from one of a first reference image or a second reference image. However, if one of the reference images is influenced by wafer noise, then the defect may not be flagged even if it is detected when comparing the test image to the other reference image.
Therefore, improved systems and techniques for defect detection are needed.